Conservative estimates suggest that amblyopia and associated deficits in binocular function effect approximately 5% of the American population. Recent studies of early visual development in cat and monkey suggest that these deficits result from abnormal visual experience, such as monocular deprivation or misalignment of the eyes, during critical periods for visual development. Until recently, lack of adequate visual assessment procedures for use with infants and pre-verbal children precluded prospective studies of visual development during critical periods in man. However, newly developed preferential-looking, operant, and electrophysiological techniques make possible research into the normal development of acuity and binocular vision and interactions between interocular acuity differences and binocular development. Prospective developmental studies will be conducted to evaluate the relationship among interocular acuity differences, hemi-field acuity differences, and binocular function in order to clarify the normal course fo visual development. These techniques will also be applied to infants and children with congenital binocular imbalance, including strabismus, anisometropia, and unilateral cataract, to evaluate prospectively the effects of early anomalous visual experience on the developing human visual system and the outcome of treatment regimens imposed at different ages during development. Comparison of visually evoked potential and preferential-looking procedures for the assessment of binocular function will provide criteria by which sensitive and appropriate procedures for monitoring binocular status during infancy may be chosen. Long-term follow-up, through the fifth year, will assess the relative benefits of various treatment modes and allow comparisons with the results from standard clinical tests. These data should help clarify the necessary and sufficient conditions for the development of normal binocular vision in man.